Heterogeneous Reaction Between Steel and Zinc Phosphate Available to Purchase

The most common anticorrosive pigments contain lead (Pb) or hexavalent chromium (Cr) compounds. These pigments are particularly hazardous and contribute to contamination of the environment. The use of zinc phosphate (ZP) has been recommended often for the formulation of environmentally compatible anticorrosive priming compositions. Ferrites and barrier pigments of lamellar structure also have been proposed. The protective action of ZP results from phosphatization of the metal substrate and the formation of complex substances with binder components to improve adhesion. However, experimental results have been contradictory. The mechanism of the anticorrosive action of ZP was studied using electrochemical tests (corrosion potential-vs-time curves, polarization curves, and corrosion rate measurements) performed in pigment suspensions and by scanning electron microscopy (SEM) analysis of steel surfaces subjected to the action of a ZP suspension or painted with an anticorrosive paint pigmented with ZP. ZP was shown to react with iron (Fe) to generate a protective oxide layer with a certain degree of crystallinity.